In many optical systems dynamic compensation of aberrated wavefronts is required to achieve optimum performance. This presents a problem, for example, in telescopes and in laser beam generation and propagation devices. In telescopes the distorted wavefronts result in poor or no image formation. In laser devices the laser beam energy becomes dispersed so that its delivery at the receiver is ineffective. To accommodate for this, closed loop compensation feedback is employed which senses a portion or sub-portion of the incoming or outgoing light beam to determine wavefront distortion. A wavefront sensor typically provides an input to a wavefront reconstructor which determines the conjugate or compensatory shape of a deformable mirror required to restore the proper wavefront shape and then through suitable drivers adjusts the actuators of the deformable mirror to the compensating shape.
Typically wavefront sensors use high resolution detector arrays such as CCDs to determine position of the centroid of the focus points resulting from a lenslet array: the position of the centroid of each focus on the CCD is a function of input wavefront distortion. But while the local tilt of the sub-portion is compensated, the relative phase of those apertures, i.e., their piston values, are not apparent. To obtain their value there are a variety of complex wavefront reconstructor algorithms in use. Each of these suffer from one or more shortcomings: such as large, heavy, slow, expensive, complex hardware, or complex and time consuming signal processing. Compensation systems which use these reconstructors operate far below the speed, 15-20 KHz which the deformable mirror actuators are capable of. Further, while CCDs can operate at a frame rate of 1000 Hz even up to 3000 Hz for the very expensive ones, the closed loop bandwidth is typically only 1/20th of that or 50-150 Hz. These complex reconstructor algorithms are required in order to stitch together the sub-apertures into a continuous surface after their individual tilts have been determined. This is so because the tilt is initially determined without respect to absolute sub-aperture piston value, i.e., the stroke position of the associated actuators.
Wavefront distortion is also a problem in light beams generated by lasers even before the beam encounters any atmospheric perturbations or other sources of wavefront distortion. Closed loop compensation is used to correct for this too. For example, when building a compound laser system to increase the total power output beyond what any one laser can provide, a number of gain cells may be operated in series. In that case wavefront distortion compensation is required at the output of each gain cell to ensure that the input to the next gain cell is proper. In co-phasing multiple lasers to achieve a single high energy beam, each individual laser would require closed loop compensation as well.
Thus far only the spatial characteristics of the aberrated wavefront have been addressed. But there are temporal considerations as well. In systems requiring multiple adaptive components, multiple feedback control systems are required. The control loop bandwidth for the subsystems must be far enough apart to eliminate cross-talk. For complex telescope systems or laser beam directors many adaptive components are required, creating a small control loop design space.